This invention relates to spreadable fluid adhesive compositions, curable by conventional vinyl free radical polymerization processes or by exposure to a source of radiation, comprising a urethane prepolymer, at least one of dicyclopentenyloxyalkyl acrylate and methacrylate, a polyol, and an effective amount of catalysts for curing of the urethane prepolymer and of the acrylate or methacrylate component. This invention is also concerned with a process for producing laminates of solid materials, for example, plastic film and metallic foil, wherein the solid materials are adhered together by the adhesive composition.
The most widely used adhesives for flexible packaging laminations are based on curable, low molecular weight polyether- and polyesterurethanes that are supplied as flammable and/or volatile solvent-borne systems. Common solvents used include methyl ethyl ketone, ethyl acetate, toluene and the like, all of which must be moisture-free to prevent premature reaction of the isocyanate groups of the polyurethane. Usually, plastic film laminates are produced by applying the solvent-borne adhesive to one film, evaporating the solvent from the applied adhesive, contacting a second film with the adhesive-bearing surface of the one film, and allowing the adhesive to cure at ambient conditions. When so used, early adhesive strength, known in the art as "green strength," is due to the high viscosity of the dry but yet uncured polyurethane adhesive. Despite the wide use of such solvent-borne polyurethane adhesives, their use is associated with several disadvantages. Process and performance limitation due to solvent content are encountered; for example, residual solvent may be retained in the final laminate thereby providing a source of odor and contamination, inks, coatings and films may be attacked by residual solvents contained in the adhesive, and elevated temperature required for evaporating the solvent may cause dimensional change in the film which results in built-in stresses in the laminate. Also, increasing solvent costs and uncertain availability are a source of increasing concern. Potential fire hazards due to the flammability of the solvents, and high insurance premiums, force serious examination and change away from solvent-borne adhesives. Still further, pollution, energy conservation, and health and safety concerns detract from the advantages of solvent-borne polyurethane adhesives.
Some improvement in the art has been achieved by the use of a solvent-free 100% reactive adhesive system wherein the solvent is replaced with conventional polymerizable ethylenically unsaturated monomers such as butyl acrylate, and the resulting adhesive composition is subsequently polymerized and cured by exposure to a source of radiation or by use of conventional free radical catalysts. This process eliminates solvent emissions but still suffers from flammability and toxicity problems and has an added difficulty due to shrinkage upon polymerization of the monomer.
Some improvement in the art has also been achieved by the use of a hot-melt adhesive system wherein a 100% solids polyurethane adhesive compound is melted to a coatable viscosity and applied to a film. While the adhesive is still molten, a second film is contacted therewith to form a laminate. The disadvantages associated with this process include the need for an expensive hot-melt applicator, the requirement to exclude moisture prior to application, and the need to supply moisture after application. The latter disadvantage can be eliminated by effecting cure by radiation.
These and other aspects of the known art relating to polyurethane adhesives for making laminates are disclosed in a paper entitled, "In Search Of The Ideal Laminating Adhesive," by Martin M. Grover and Stuart H. Ganslaw, in "Paper, Film and Foil Convertor," November, 1977, page 71.